Relative humidity meter



Nov. 12, 1946'.

F. KAHN 2,411,041 RELATIVE HUMIDIT-Y METER FiledAug. 7, 1942 2 Sheets-Sheet l 57 t 7l il 40 3l u :s b mlllmlnulmllnnl llnullmhm $6 50 l y f 4+ Fay. .5f

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INVENTOR 5 la ls n a l f- M or "'afrfgfymWzzzfmf Nov. l2, w46. F. KAHN RELATIVE HUMIDITY METER Filed Aug. '7. 1942 2 Sheets-Sheet 2 INVENTOR I Patented Nov. l2, 1946l Y UNITED STATES PATENT. OFFICE RELATIVE HUMmi'rY METER Frank Kahn, Philadelphia, Pa. Application August 7, 1942, serial N6. 454,009 12 claims. (ci '1s-33s) The present invention is a continuation-impart of my patent application, Serial No. 229,281, led September 10, 1938, now Patent No. 2,293,064, datedAugust 18, 1942, relating to relative humidity meters.`

The invention relates to hygrmetry, and, more particularly, relates to a direct reading relative humidity meter. v

' The most convenient and most generally employed method for measuring the degree of saturation ofthe atmosphere with moisture is to observe the temperature of evaporation, that is, the di'erence between the dry-air and AWet-air temperatures. The dry-air or ambient temperature is read with the ordinary thermometer. The Wet-air temperature may be accurately observed with the sling or whirled psychrometer, with the wet-air thermocouple described in Patent No. 2,128,462, dated August '30, 1938, issued jointly to Edward R. Wayne and myself, and relating to thermocouple hygrometers, and with my bimetal wet-air 'temperature responsive unit hereinafter described in detail. With the difference between the dry-air and wet-air temperatures determined, the relative humidity may be obtained from the Weather Bureau Psychrometric Tables, or the .equivalent standard formula or chart.A

This method of determining relative humidity,

p although of high accuracy, is inconvenient and because it is not direct readingis unsuited for recording or for control of humidity conditioning apparatus. On the other hand`reasonably priced reading relative humidity indicator of good accuracy direct reading devices, which have heretofore'- been based upon relative tension of hy'groscopic materials, have been highly unreliable, and have Y required much too frequent calibrations and adjustments to render their use practical where any degree of accuracy is required.

'The increasing importance of air conditioning emphasizes the need for a simple, compact, inexpensive and reliable device for measuring relative humidity. Apparatus for` this purpose heretofore available has been extremely large and expensive, ,very inconvenient and laborious to operate, or highly unreliable. f

The primary object of my invention is to provide a, direct reading relative humidity meter in which these defects are remedied.

An object of'my invention is to produce a direct reading relative humidity meter of high accuracy. Another object of my invention is to provide a reliable direct reading relative humidity recorder. A further object of -my invention is to produce an inexpensive, simple, durable and reliable direct Another object oi my invention is to provide anindicating instrument having an expansible scale.

Still another object of my invention :is to provide a bimetal wet-air temperature responsive unit. v

Still a further object of my invention is to provide an automatic direct-reading relative humidity meter which is accurate, occupies a minimum of space, requires very littlewater, and needs l practically no attention or servicing.

With these and other objects in view, which will become apparent as the description proceeds, my invention is embodied in a relative .humidity observing system which consists essentially of a wet-air temperature responsive'unit in a stream of the airbeing metered, a dry-air temperature responsive unit in the same air, a meter having a movable element arranged in cooperative relation with both said Wetand dry-air units and adapted to be deiiected proportionally tothe difference in response of said units,vand means responsive tothe dry-air temperature of the air being measured to cause the indication of the meter to vary as a function of the dry-air temperature.

'Ihe invention, both as to details of construction and combination of parts, will best be understood from the following description of a speciiic embodiment which I illustrate as an example, when read in conjunction with the accompanying drawings, in which:

Fig. 1 is a diagrammatic representation of a relative humidity responsive single thermocouple circuit powering the movable element of the meter of my invention.

Fig. 2 is a' diagrammatic representation of a relative humidity responsive thermopile circuit Fig. 4 is a diagrammatic view of -my invention showing the meter with resilient scale.

Fig. 5 is a diagrammatic view of a modification of the resilient scale.

Fig. 6 is a front elevation of the self-contained direct-reading relative humidity meter of my v invention. y

Fig. 7 isa fragmentary front elevation of the device oi Fig. 6.

Fig. 8 is a perspective view of the wet-air uni of the device of Fig. 6.

Fig. 9 is a fragmentary side elevation of a modification of the device of Fig. 6 in which the electric fan motor is replaced by a spring motor.

Fig. 10 is a fragmentary perspective view, partly meter powered by opposed bimetal elements.

Fig. 11 is 9, fragmentary perspective, partly in section, of a modification of my meter having dry-air temperature responsive deflection sensitivity.

Fig. 12 is a fragmentary sectional view on the une' Iz-lz of Fig. 11.

Fig. 13 is a diagrammatic view of my meter with non-resilient proportionally variable scale.

Referring now to the drawings, Fig. 3 is a graph in which the unbroken lines are curves of constant relative humidity, ordinates are dry-air or ambient temperatures, and abscissae are depressions of wet-air temperature below dry-air temperature. These data are plotted from the standard ps ychrometric tables in the United States Weather Bureau publication W. B. No. 235. l

Corresponding respectively to the unbroken curves of constant relative humidity, I have shown an equal pluralityof broken straight lines I8 diverging from a common point I9 on the axis of ordinates at about eight degrees Fahrenheit, one of said'straight lines being drawn to coincide generally with ach of the constant relative humidity curves betweenthe ordinates 40 F. and 100 F. As is readily evident from Fig. 3, per cent relative humidity can thus be represented with good fidelity by a series of straight lines on a graph of dry-air temperature against depression of wet-airtemperature.

'An indicating instrument having a deflecting element responsive to difference between wet-air and dry-air temperatures, `when calibrated at constant dry-air temperature in accordance with the intersections o'f the relative humidity lines I8 with the ordinate representing the dry-air temperature, will indicate with good accuracy per cent relativehumidity at that temperature. If the deflection of the element relative to the scale for a given wet-air to dry-air temperature differential be then made to vary inversely as the dry-air temperature less approximately 8 F., the instrument will indicate percentage relative humidity with good accuracyover'the range of usual ambient temperatures as illustrated in Fig. '3. Inother words, this effect is produced by merely making the sensitivity of indication of the meter inversely proportional to the dry-air temperature above approximately 8 F. I use the term sensitivity of indication to express the degree to whichthe'deflection of the element relative to the scale is responsive to the difference between `the lwet-air and dry-air temperatures. As an illustration, referring to Fig. 3, the reading or indication for a dry-air temperature of 80 and a diierential wet-air to dry-air'temperature in section,.showing the movable element of myV ing a thermojunction 2| formed by union-of two couple elements 22 and 23 secured with good thermal contact to a thermally conducting meml ltype ammeter 26, which forms the reference junction. When the air to be measured is forced past .the wet-air unit 20 at the proper 'velocity with the wick 25 suitably moistened and with the ammeter 26 at the dry-air temperature, the net M. F. of the circuit, and therefore the current in the circuit, is substantially proportional tothe depression of the wet-air temperature below the dry-air temperature.

Greater currentfor the same wet-air depression may beobtained with the wet-air unit of Fig. 2, and thus make possible the use of a `much less In place of the element 24 of Fig. 1, there are shown in Fig. 2, three elements 30, each the functional equivalent of element 24. The thermocouples 3| are arranged as a thermopile with intermediate junctions 32 at the same temperature as the reference junction at .the ammeter 33. The elements and attached thermocouples are electrically insulated from each other by shellac or otherelectric insulating coating which will not materially reduce the heat transfer from the elements 30 to the evaporative envelope 34.

One method of providing the desired variation of sensitivity of indication with change of ambient temperature is illustrated in Fig..4 in which a dArsonval ammeter 35 is shown comprised of a permanent magnet 36, core 31, movable coil 38 and lpointer 39. In place of the'customary scale, however, there is provided a resilient scale 40 composed of a flat rubber band, or the like,

graduated in per cent relative humidity with the spacings of the graduations proportional to the horizontal spacings of the broken relative humidity lines of Fig. 3. The left hand end 4I of the scale 4I| is fixed relative to magnet 36, while the right hand end 42 is attached to the temperature responsive or movable end 43 of a bimetal element 44 whose other end 45 is also fixed relative to magnet 36. When the ammeter 35 is suitably connected to one of the wet-air units ypreviously described, the pointer 39 will deflect proportionally to the depression of the wet-air temperature below ambient, or proportionally to f vthe abscissae of the graph of Fig. 3. The element of 20 F. would be the same as for a dry-air tem- Y perature of 40 F. and a differential temperature between wet-air and dry-air temperatures. Fig.

1 shows a wet-air thermocouple unit 20 compris- 44 is constructed so that the movement of its end 43, and therefore the stretchl of the resilient scale- 40, in response to changes in ambient temperature, is such that for any ambient temperature between .40 F. and F., the scale 40 will be stretched to correspond with thebroken lines of the graph of Fig. 3 at the particular ambient temperature ordinate. Because the broken linesl I8 are straight and converge to a point this is a simple matter. The departure in Fig. 3,V of, the broken lines vfrom the unbroken lines, between the ordinates 40 F.- and-100 F., shows that the.

errors to be. expected in reading this meter directly inrelative humidity are of a very low order. A pointer 46 is attached to the end 43 of birnetal element 44 and is arranged to traverse a scale 41 fixed with respect to magnet 36 and suitably graduated in degrees of temperature so that the device also functions as a thermometer.

In place of the at band 40, I have shown in the relative humidity meter 56 of Fig-5, an elastic scale 5I composed of a attened helical spring 52. The major or cardinal divisions are proof scale vided with index tabs 53 extending beyond the spring 52 and being marked with the appropriate value of per cent relative humidity.

A stitl' internal core 55, suitably secured in fixed relationship to the frame 58 of the meter, supports the spring 52 in the desired scale shape, in this case illustrated as a circular arc. The left hand end 51 of the scale 5| is secured to core 55. IA bimetal element 44 has one end 45' fixed to the frame 58, with the other end 43' secured to the movable end 58 ofthe scale 5|. 'I he separations oi.' the individual turns of spring 52 are preformed to correspond to aliquot subdivisions of per cent relative humidity in accordance with the separations of the broken lines of the graph of Fig. 3, taking into account the shape A pointer 59 attached to the end 58 of scale 5| registers with an ambient temperature scale 60 fixed to the core 55V so that the meter can also be used as a thermometer.

My invention, as embodied in a. small selfcontained compact portable relative humidity indlcator, is illustrated in Figs. 6, 7 and 8. A base 85 has attached theretoa miniature centrifugal blower 68 driven by an electric motor 61 which is supplied with electricity via leads 68 and 69 from a suitable source of current supply (not illustrated) which may be A.C. or D.C. A collar bracket is provided with an outward flange 1| which is attached by screws to the base 65. A container 12 having a cylindrical lower reservoir section 13, a duct section 14,l and a mouth section 15, is held in 'place by the bracket 10 which fits about .the reservoir 13; cates with the reservoir 13 by way of an opening 18 rimmed by an annular shoulder 11. the mouth 15 being centered over the opening 18. A small wet-air thermocouple unit 18 which maybe of, the type shown in Figs. l or 2 or the equivalent f.

thereof, but illustrated as of the thermopile type of Fig. 2, has its conductor elements 19 extending through acork or rubber stopper 80 which iits snugly into the mouth 15 so that the body of the wet-air unit 18 is suspended centrally within the duct 14 and the dependent wick 8| of the unit 18 passes through the opening 16 into the reservoir 13. A conical collar 82 of waxed paper or other non-absorbent material, is fitted over the wick 8| so that said collar rests on the shoulder 11, thereby sealing reservoir 13 and minimizing the rate at which water is evaporated therefrom.

The exhaust vent 83 of the fan 66 abuts the inlet end of the duct 1I and the joint is made airtight with an elastic sleeve 84 which fits over both parts. A dArsonval meter 85 constructed according to the modifications of my invention hereinbefore described or those disclosed in my aforesaid copending patent application Serial No.

The duct 14 communi- Vsleeve A |05 is insignificant.

cent relative humidity in accordance with the broken lines of the graph of Fig. 3, as previously explained.

The illustrations of Figs. 6 and 'l depict .the

container 12 as made of glass but it may be made of any other suitable material `such as metal or a. resin. In place of the electric motor 81, a spring powered motor 90 with manual wind 9| on a base 65 illustrated in Fig. 9, may be used to' drive the ian 56 which is shown with a screen or filter 92 to minimize accumulation of dirt in the duct or on the wet-air unit, although experiencevhas shown that the wet-air unit will operate with good accuracy when quite dirty. The meter is designed for both continuous operation and for spot readings. In the latter case the fan is started and the indicating pointer of the meter is observed until it reaches a minimum value of relative humidity, the steady state condition being reached in a few seconds as a result of the small size of the wet-air unit. Either of the wetair units illustrated in Figs. l or 2 may be used with this meter, a less sensitive ammeter being required if the wet-air thermopile of Fig. 2 is used.

In place of the thermocouple wet-air units of Figs. 1 and 2, the bimetal wet-air unit |00 of the meter shown in Fig. 10 may be used. In this device a bimetal spiral |0| has its internal end xed to a concentric shaft |02 rotatably mounted in a ball thrust bearing |03. The external end of the spiral |0| is attached to a rigid arm |00 `fixed to a cylindrical sleeve |05 integral with or secured to the stationary part of the bearing |03. The shaft |02 and the sleeve |05 are made of thermally non-conducting material such as Synthetic resin or plastics. An evaporative envelope` |08 completely encloses the spiral |0| ard a part of lthe sleeve |05, in such manner that heat leakage into the interior of said envelope |00 via the 'I'he wet-air unit |00 comprises the envelope |08 and the enclosed therein.

The shaft |02 extends beyond the bearing |03 and carries at the outer end a bimetal spiral |01 similar to the spiral |0| and similarlyl attached to the shaft |02. The external end of the spiral anism is mounted so that the upper or indicating 229,281 as being adapted to read directly in per cent relative humidity when connected to a properly conditioned wet-air thermocouple unit, is also attached to the base 55, the terminal leads 88 and81 oi' Ithe wet-air unit 18 being connected thereto.

'Ihe operation of this meter is as follows. The reservoir 13 is lled with water 88 through the mouth 15 and opening 18. The assembly comprising stopper 80, wet-air unit 18, wick 8|, and collar 82 is slippedinto place through the mouth 15. Electric current is then supplied to the motor 51 causing the fan 58 .to'force a stream of air through the duct 14 over 'the unit 18. In a few seconds the temperature of the measuring junction of the unit 18 will be reduced to the' wet-air temperature and the meter 85 will indicate per end I|2 of pointer |08 traverses the expansible scale 40' which may be any of the scales shown in Figs. 4, 5, 13 or their equivalents.

When the wet-air unit |00 and the spiral |01 are exposed to a current of the air to be measured at the proper velocity, the spiral |0| will soon reach the wet-air temperature and the spiral |01 the dry-air temperature. The pointer |08 will be rotated by the member |09 to a. posi-I tion corresponding to the differential rotation of the spirals |0| and |01 which is proportional to the difference between the wetand dry-air temperatures or the abscissae of the graph of Fig. 3. In effect this device is a mechanical functional equivalent of the thermocouple circuits of Figs. 1 and 2.

vIn Fig. 11 I have shown the wet-air unit |00'V which is similar to the unit |00 except that the axis of rotation is vertical instead of horizontal and the spiral |0| is encased in a housing H5 of thinmetal of good thermal conductivity such as copper or silver. The housing ||5 prevents the envelope |00 from interfering with the movemechanism vferential unit illustrated.

perature. A crank I I6 journaled in a fixed bearing aligned with shaft |02 has its oil'set end attached to the external end of the dry-air spiral.

|01. Another bearing I|8 is translatably mounted in a guide ||9 for constrained movement in alignment with shaft |02 as imposed by a U-shaped bimetal element |22 having its-ends attached respectively to the bearings I Il and I3. A shaft |2| is journaled in bearing lI I8 which is of the thrust type to prevent axial movement of the shaft. The shaft |2| is provided with a slotted sleeve |22 at its lower end to receive the upper end of the crank shaft H which carries a transverse pin |23 engageable inthe slot toY prevent relative rotation of the two shafts while permittingv relative axial movement. The upper end of shaft |2I is bent at right angles to form a lever |24 whose end engages a radial slot |25 in the upper end of a gear sector |25.

The sector |26 is fixed to a shaft |27 Journaled in a fixed bearing 28, theishaft |21 being attached to the sector at a point between the slot |25 and the lower or gear end I 29 so that the axis of rotation of the sector is relatively very close to the slot |25'as compared with the end |29. The gear |29 engages a pinion |30 fixed to a. shaft |3| which extends through and is journaled in a bearing |32 and carries a pointer |33 operating over a xed scale |3d. A ne spiral spring |35 is attached to shaft |3| and bearing |32 to eliminate backlash.

This device functions as follows: The shaft IIB rotates proportionally to the difference between wetand dry-air temperatures as previously explained for the device of Fig.- 10. This rotation isy communicated through shaft |2| to the sector |26, but the point of engagement of the lever |24 in thevslot |25 is determined by the bimetal element |20, That is, the higher the ambient teni'- perature, the higher the bearing ||8 will ascend in its guide I I9 and the less will be the rotation of the sector |26 and hence of the pointer |33. The object of the proportioning of the sector |26 to give a high mechanical advantage of the order `of V to 1 or more is to provide a relatively great movement of the pointer |33 for avery small deflection of the lever |24 in the slot' |25. Not only does this construction eliminate the need for excess clearances in the slot and lever connection as shown in Fig. 12, but the proportionality of angular movement is maintained at that connection because only very small angles are involved. In this device it isobvious that the shaft IIE could be operated from the rotating elements of the ammeters of the therm'ocouple circuits of Figs. 1 and 2 as well as from the mechanical dif- 'I'he principle 9! the elastic scale in connection with my relative humidity meter is not intended to be limited to thev use of an elastic medium. In place of the elastic or resilientv scales of Figs.

4 and 5, the scale may be constructed in the manner of a folding fan or it may be constructed sim.. ilarly to the iris of a camera; with the component elements representing major divisions of per cent relative humidity and arranged to be positioned in accordance with the graph of Fig. 3

for the various ambient temperatures by a suitable bimetal element or the equivalent, An example of such construction is illustrated in Fig. 13 in which arms |90 pivoted at a point |9| are aardbeiy 8 positioned in accordance with the lines I8 of Fig. 3 to represent the cardinal percentages of a relative humidity scale as indicated by the index tabs 53'. The positioning is accomplished by the bimetal element 44" which revolves a suitably guided circular-arc rack |32 about the point |9| as the ambient' temperature changes. Proportional changes in spacing of the arms |90 are obtained by means of pinions|93 rotatably attached engage both the rack |92 and individual sectors of a fixed annular gear led. The individual pin, ions |93 and sectors of gear |94 are severally proportioned to move the arms |39 to the desired positions for any particular ambient temperature.

For intermediate scale divisions, elements of wire or other suitable material may be attached to'or be guided by the median intersections of lazy tongs connected between pairs of the arms |9. This construction is shown in Fig. 13 for scale subdivisions between 10% and 20% relative humidity, the elements |95 representing scale subdivisions being positioned by the two sets of lazy tongs |96 and |91. The space enclosed in the dashed lines is the portion of the scale intended to be exposed to view in the assembled meter. The indicating element for this device may be that of Figs. l, 2, 10 or their equivalents.

It should be realized that the foregoing ther- 30 moelectric powered meters are suitable for remote indication of relative humidity. It is wellknown that in place of the dry-air responsive thermosensitive elements, generally shown herein as bimetal devices, remotely actuated thermosensitive devices, such as that illustrated in Fig. 6 of the aforesaid Patent No. 2,293,064 or its functional equivalent may be used. Suchvconstruc- Vtion permits the meter in its various forms to be used for air in ducts or other inaccessible places and the location of the meter itself to suit the convenience of the user.

Although the compact self-contained device of Figs. 6 and 7 is shown with a thermoelectric powered differential unit, it is obvious that the bimetal powered dierential units of Figs. l0 and 11 maybe used instead with either the expansible scale or the variable deflection sensitivity control.

Although the invention has been described in considerable detail, such description is intended as illustrative rather than limiting, as numerous4 embodiments will be apparent to those skilled in the art. My invention, therefore, is not to belimited except insofar as is necessitated by the prior art or by the spirit of the appended claims.

I claim: i 1. In a direct reading relative humidity meter, a deecting element responsive to the difference between the wet-air and dry-air temperatures of the air to be measured, a scale adapted to be varied in length, a pointer adjacent to said scale and constructed and arranged to be deilected over said scale proportionally to the deection of said element, and means to vary the length of the 5 scale such that the sensitivity of indication of said pointer relative to said scale is in inverse proportion to the dry-air temperature in degrees F. less approximately 8 F.

2. In a direct reading relative humidity meter. a differential unit including a pointer responsive to the difference between the wet-air and the dryair temperatures of the air to be measured, a scale adjacent to the path of said pointer, said scale being adapted to be varied in length and the divisions of said scale to beproportionally clon-- to the outer ends of the arms so that they` gated vor contracted as the scale is lengthened orl shortened. and means to vary the length of said 4. In an indicating instrument, a frame, a scale,

a 'deflection element mounted on said frame and closed within said envelope, a xed cylindrical sleeve member piercing said envelope, a. shaft disposed coaxially within said member, a second bimetal spiral disposed externally to said envelope,

, the inner end of each of said spirals being respechaving a pointer adjacent saidscale and movable with respect4 thereto, said scalehaving one end fixed to said frame and being .adapted -to be elastically stretched, and means for stretching said scale. said means being mounted on said frame independently of said element.

5. In a hygrometer, a wet-air unit comprising an evaporative envelope adapted tobe subjected to standard evaporating conditions, a bimetal spiral enclosed within said envelope, a sleeve member piercing said envelope and having a cylindrical opening, a shaft rotatablyA disposed in the opening in said member. said shaft being fixed to the inner end of said spiral and said member being iixed relative to the outer end of said spiral.

6, The invention set forth in claim 5 characterized in that said member and said shaft are composed of thermally non-conducting material, and said spiral being proportioned to the evaporative properties of the envelope such that said spiral is cooled substantially to the temperature of the surface of the envelope.

7. The invention set forth in claim 5 characterized in that said spiral is enclosed withina housing of thin metalof good thermal conductivity.

8. In a relative humidity meter, a diierential wetand dry-air unit comprising an evaporative tively fixed to each end of said shaft and coaxial therewith and the outer end of said rst spiral being ilxed relative to said member, and means to support said shaft rotatably within said member whereby the outer end of said second Spiral iS deflected in response to the diiierence between the wetand dry-air temperatures of the air being measured.

9. In a direct reading relative humidity meter, the combination of a thermocouple circuit comprising (1) a wet-air thermocouple unit adapted to be exposed under standard evaporating conditions to the air to be measured, (2) a reference thermojunction in said air. and (3) an electric instrument having an indicating element responsive to the current in said circuit. said instrument including a scale adapted to be varied in length.

and means for varying the length of said scale asa function of temperature.

10. The invention set forth in claim 9` characterized in that the divisions of said scale are 'adapted to beproportionally elongated and contracted as the scale is lengthened and shortened.

11. 'I'he invention set forth in claim 9 charactermed in that said scale is elastic and that said' means is adapted to stretch said scale.

12. The invention set forth in claim 2 characterizedin that said meterincludes means for directing a current of the air to be measured against the wet-air component of said diierential unit, and reservoir means for supplying moisture to said wet-air component.

FRANK KAHN. 

